As miniaturization continues to be an industry goal, technological advances in integrated circuit design and PCB (Printed Circuit Board) design have provided circuit engineers the ability to manufacture denser circuit boards using advanced fine-pitch interconnect technology. Examples of advanced fine-pitch interconnect technology include electrical components fabricated from BGA (Ball Grid Array) technology or DCA (Direct Chip Attach) technology, just to mention a few. To employ fine-pitch technology, however, manufacturers have had to improve PCB substrates as well. An example of such an improvement is HDI (High Density Interconnect) technology.
These advances have given rise to new challenges--principally, in the area of interconnect reliability as measured under average to extreme environmental conditions. FIGS. 1-5 illustrate how present manufacturing technologies in board and IC design have created reliability issues not yet solved in the present art. FIG. 1 depicts the interconnect between a conventional BGA component 102 and a conventional PCB substrate 110. The BGA component 102 includes one or more bondable elements shown as the combination of a stud 104 coupled to a solder bump 106. During the manufacturing process, the bondable elements are aligned with a corresponding set of bondable pads 108 on the PCB 110, which provide electrical interconnect to other components by way of conventional vias and runners connected thereto. FIG. 2 illustrates the PCB 110 and BGA 102 after both have been subjected to, for example, a conventional reflow process.
FIGS. 3-4 show perspective views of the PCB 110 and the bondable pads 106 before and after the PCB 110 is subjected to forces generally encountered in the field as a matter of consumer use. As FIG. 3 illustrates, before any forces are applied to the PCB 110, the surface of the PCB 110 is relatively flat. Under these environmental conditions, interconnects consisting of the bondable pads 106 and the bondable elements (not directly shown in FIGS. 3-4) are free from stress. FIG. 4 illustrates what happens when one or more forces representative of a predetermined maximum stress (e.g., forces encountered during a drop test) are applied to a circuit carried by the PCB 110. Note that although this figure illustrates uniformly distributed forces applied on a side of the PCB 110, it will be appreciated that realistically during a drop test, or like stress test, such forces are distributed non-uniformly throughout the PCB 110. In FIG. 4, the highest stress points are found in corners of the array of bondable pads 108. The bondable pads 108 in the corners tend to encounter more stress because their position on the PCB 110 subjects them to multidirectional forces 112, 114, which the other pads are not subject to.
With fine-pitch technology such as BGA, the dimension of the solder bump 106 is substantially reduced to accommodate the high density of interconnects. As a result of the smaller dimension, the strength of the interconnect between the solder bump 106 and the bondable pads 108 is substantially reduced when compared to older interconnect technologies using larger dimensions. As a result of a weak bond, interconnects are vulnerable to electromechanical disconnect such as, for example, fractures 118 as shown in FIG. 5. Although the bondable pads 108 nearest the stress forces shown in FIG. 4 may result in fractures, the corner pads 108 have a higher likelihood of fracturing because of the multidirectional forces 112-114 these pads encounter.
Presently, this deficiency in the prior art has produced field failures in portable selective call radios such as, for example, cellular phones, and pagers, which are often subjected to harsh environments. Accordingly, a need exists for a method that improves the reliability of electromechanical interconnects in selective call radios utilizing advanced PCB interconnect technologies.